U.S. patent number 4,705,620 [Application Number 06/942,147] was granted by the patent office on 1987-11-10 for mercaptan extraction process.
This patent grant is currently assigned to UOP Inc.. Invention is credited to Jeffery C. Bricker, Bruce E. Staehle.
United States Patent |
4,705,620 |
Bricker , et al. |
November 10, 1987 |
Mercaptan extraction process
Abstract
A process is disclosed for treating a sour hydrocarbon stream
comprising extracting the mercaptans contained in said hydrocarbon
stream with an alkaline solution in an extraction zone, oxidizing
the mercaptans to disulfides in the presence of an oxidation
catalyst, separating said disulfide from said alkaline solution,
reducing the residual disulfides in said alkaline solution to
mercaptans and recycling said alkaline solution to the extraction
zone. Two ways are disclosed to effect the reduction of the
disulfides to mercaptans: (1) hydrogenation with a supported metal
catalyst and (2) electrochemical reduction.
Inventors: |
Bricker; Jeffery C. (Mt.
Prospect, IL), Staehle; Bruce E. (Buffalo Grove, IL) |
Assignee: |
UOP Inc. (Des Plaines,
IL)
|
Family
ID: |
25477639 |
Appl.
No.: |
06/942,147 |
Filed: |
December 16, 1986 |
Current U.S.
Class: |
208/206; 208/226;
205/444; 208/235 |
Current CPC
Class: |
C10G
19/02 (20130101); C10G 19/08 (20130101) |
Current International
Class: |
C10G
19/00 (20060101); C10G 19/02 (20060101); C10G
19/08 (20060101); C10G 019/00 (); C10G 019/08 ();
C10G 045/00 () |
Field of
Search: |
;204/73R ;210/757
;208/226,235,206 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Metz; Andrew
Assistant Examiner: Myers; Helane
Attorney, Agent or Firm: McBride; Thomas K. Spears, Jr.;
John F.
Claims
We claim as our invention:
1. A process for treating a sour hydrocarbon stream containing
mercaptans which comprises:
(a) contacting said hydrocarbon stream with an aqueous alkaline
solution in an extraction zone at treating conditions to form a
purified hydrocarbon stream and a mercaptide rich aqueous alkaline
solution;
(b) separating and recovering said purified hydrocarbon stream from
said mercaptide rich aqueous alkaline solution:
(c) passing said mercaptide rich aqueous alkaline solution to an
oxidation zone and therein treating said mercaptide rich aqueous
alkaline solution with an oxidizing agent in the presence of a
metal phthalocyanine oxidation catalyst at oxidation conditions to
oxidize the mercaptides to liquid disulfides;
(d) separating a major portion of said liquid disulfides from said
aqueous alkaline solution which contains residual disulfides in a
separation zone;
(e) passing said residual disulfide containing aqueous alkaline
solution to a reduction zone and reducing said residual disulfides
to mercaptans at reduction conditions; and,
(f) recycling said mercaptan containing aqueous alkaline solution
to said extraction zone.
2. The process of claim 1 in which said hydrocarbon stream
comprises light paraffin gases (C.sub.1 -C.sub.4 hydrocarbon).
3. The process of claim 1 in which said hydrocarbon stream
comprises light naphtha (C.sub.4 -C.sub.6 hydrocarbon).
4. The process of claim 1 in which said treating conditions
comprise a temperature from about 10.degree. to about 100.degree.
C. and a pressure from about ambient to about 300 psig.
5. The process of claim 1 in which said oxidation conditions
comprise a temperature in the range of from about 35.degree. to
about 70.degree. C., a pressure in the range of from about ambient
to about 100 psig and an air concentration from about
stoichiometric to about 1.5 the stoichiometric amount.
6. The process of claim 1 in which said reduction is effected in
the presence of a hydrogenation catalyst, a hydrogen concentration
in the range of from about 10 to about 100, a temperature in the
range of from about 40.degree. C. to about 100.degree. C. and a
pressure in the range of from about 50 to about 125 psig.
7. The process of claim 1 in which said reduction is effected in an
electrochemical cell consisting of an active electrode and a
counter electrode such that the disulfides are electrochemically
reduced to mercaptans.
8. The process of claim 7 in which the active electrode is further
characterized as being selected from the group comprising zinc,
lead, platinum, graphite, glossy carbon, carbon, cadmium,
palladium, iron, nickel and copper.
9. The process of claim 7 in which the counter electrode is further
characterized as comprising platimun.
10. The process of claim 7 in which the counter electrode is
further characterized as comprising graphite.
11. The process of claim 6 in which said hydrogenation catalyst is
further characterized as comprising from about 0.01 to about 5 wt.
% palladium supported on carbon.
12. The process of claim 6 in which said hydrogenation catalyst is
further characterized as comprising from about 0.1 to about 8 wt. %
platinum supported on carbon.
13. The process of claim 6 in which said hydrogenation catalyst is
characterized as comprising from about 0.1 to about 8 wt. % nickel
supported on alumina.
14. The process of claim 6 in which said hydrogenation catalyst is
further characterized as comprising a Group VIII metal carboxylate
and is present in the alkaline solution.
15. The process of claim 14 in which said metal carboxylate is
further characterized as a palladium carboxylate.
16. The process of claim 14 in which said metal carboxylate is
further characterized as comprising a nickel carboxylate.
17. The process of claim 1 in which said alkaline solution is
either sodium hydroxide or potassium hydroxide.
18. The process of claim 12 in which said alkaline solution is
further characterized as having a pH in the range of from about a
pH of 8 to about a pH of 14.
19. The process of claim 1 in which said metal phthalocyanine
catalyst is selected from the group comprising a Group VIII metal
phthalocyanine sulfonate.
20. The process of claim 18 in which said metal phthalocyanine
sulfonate is further characterized as comprising cobalt
phthalocyanine sulfonate.
21. The process of claim 18 in which said metal phthlocyanine
sulfonate is further characterized as comprising iron
phthalocyanine sulfonate.
22. The process of claim 18 in which said metal phthalocyanine
sulfonate is present as a dissolved or suspended solid in the
alkaline stream.
23. The process of claim 18 in which said metal phthalocyanine
sulfonate is supported on a suitable carrier material.
24. The process of claim 23 in which said carrier material
comprises activated charcoals.
25. The process of claim 1 in which said oxidizing agent is
oxygen.
26. The process of claim 1 in which said oxidizing agent is air.
Description
BACKGROUND OF THE INVENTION
Traditionally the removal of mercaptans from various process
materials and/or streams has been a substantial problem. The
reasons for desiring this removal are well-known in the art and
include: corrosion problems, burning problems, catalytic poisoning
problems, undesired side reaction problems, offensive odor
problems, etc.
The methods that have been proposed for the solution of this
removal problem can be catagorized into those that seek the
absolute removal of mercaptan compounds or any derivatives of these
compounds from the carrier stream or material, and those that seek
only to convert the mercaptans into a less harmful derivative with
no attendant attempt at removal of these less harmful derivatives.
Solutions of the former type are generally labeled as "extraction"
processes. Solutions of the latter type are generally labeled as
"sweetening" processes. Prominent among the extraction processes is
a process which depends for its effectiveness on the fact that
mercaptans are slightly acidic in nature and in the presence of a
strong base tend to form salts--called mercaptides--which have a
remarkably high preferential solubility in a basic solution. In
this type of process, an extraction step is coupled with a
regeneration step and an alkaline stream is continuously
recirculated therebetween. In the extraction step, the alkaline
stream is used to extract mercaptans from the hydrocarbon stream,
and the resulting mercaptide rich alkaline stream is treated in the
regeneration step to remove mercaptide compounds therefrom with
continuous cycling of the alkaline stream between the extraction
step and the regeneration step. The regeneration step is typically
operated to produce disulfide compounds which are immiscible in the
alkaline stream, and the major portion of which is typically
separated therefrom in a settling step. In many cases, however, it
is desired to remove substantially all disulfide compounds from the
alkaline streams and complete separation of disulfide compounds
from the alkaline stream in a settling step is not feasible because
of the high dispersion of these compounds throughout the alkaline
solution. Accordingly, the art has resorted to a number of
sophisticated techniques in order to coalesce the disulfide
compounds and effect their removal from the regenerated alkaline
solution. One technique that has been utilized involves the use of
a coalescing agent such as steel wool in order to spring disulfides
from the regenerated alkaline solution. This technique, however,
results in significant amounts of disulfides left in the alkaline
solution. Another technique which has been widely utilized involves
the use of one or more stages of a naphtha wash (see for example
U.S. Pat. No. 3,574,093) in order to extract disulfide compounds
from this alkaline solution. This technique has been widely
utilized in the art, but it has several disadvantages: (1) it
requires the availability of naphtha; (2) it requires large volumes
of naphtha because of its low efficiency; (3) it requires a
separate train of vessels and separators; and (4) it requires
disposal of the contaminated naphtha.
As is well known to those skilled in the art, there are certain low
boiling range hydrocarbon streams for which it is absolutely
critical that the amount of sulfur compounds contained therein be
held to a very low level. In many cases, this requirement is
expressed as a limitation on the total amount of sulfur that can be
tolerated in the treated stream--typically the requirement is for a
sulfur content less than 50 wt. ppm calculated as elemental sulfur,
and more frequently, the requirement is less than 10 wt. ppm
sulfur. Accordingly, when a mercaptan extraction process of the
type described above is designed to meet these stringent sulfur
limitations, it is essential that the amount of disulfides
contained in the regenerated alkaline solution be held to an
extremely low level in order to avoid contamination of the
extracted stream with disulfides. For example, in the sweetening of
a hydrocarbon stream containing C.sub.3 and C.sub.4 hydrocarbons
and about 750 wt. ppm mercaptan sulfur, an extraction process can
easily be designed to produce a treated hydrocarbon distillate
having about 5 wt. ppm mercaptan sulfur; however, without special
treatment of the regenerated alkaline solution utilized, the total
sulfur content of the treated hydrocarbon stream will be about 50
wt. parts per million because of disulfide compounds which are
returned to the extraction step via the alkaline stream where they
are transferred to the treated hydrocarbon stream.
The instant invention cures this problem by treating the disulfide
containing alkaline solution in a reduction step whereby the
disulfides are reduced back to mercaptans. Since the mercaptans are
preferentially soluble in the alkaline phase, they are not
transferred to the treated hydrocarbon stream. The reduction of
disulfides to mercaptans is known in the art but is carried out for
other purposes than that presented herein (See U.S. Pat. No.
4,072,584). Reduction of the disulfide can be accomplished by
either hydrogenation of the disulfide with hydrogen over a
hydrogenation catalyst or by electrochemical means wherein the
disulfide is reduced at the cathode electrode of an electrochemical
cell. Some of the broad advantages associated with this solution to
the sulfur reentry problem are: (1) it eliminates the disposal
problem and additional separation hardware required for naphtha
washing; and (2) it minimizes the amount of mercaptides in the
alkaline recycle stream charged to the extraction zone.
SUMMARY OF THE INVENTION
This invention relates to a process for continuously treating a
sour hydrocarbon stream containing mercaptans in order to generate
a purified stream of reduced mercaptan content and of reduced total
sulfur compound content. More precisely, the present invention
relates to a process for the treatment of a sour hydrocarbon
fraction for the purpose of physically removing mercaptans
contained therein which process comprises extracting the mercaptans
in an extraction zone with an alkaline solution, oxidizing the
mercaptans to disulfides in the presence of an oxidation catalyst,
separating said disulfide from said alkaline solution, reducing the
residual disulfides in said alkaline solution to mercaptans and
recycling said alkaline solution to the extraction zone.
Accordingly, one embodiment of this invention provides a process
for treating a sour hydrocabon stream containing mercaptans which
comprises:
(a) contacting said hydrocarbon stream with an aqueous alkaline
solution in an extraction zone at treating conditions to form a
purified hydrocarbon stream and a mercaptide rich aqueous alkaline
solution;
(b) separating and recovering said purified hydrocarbon stream from
said mercaptide rich aqueous alkaline solution;
(c) passing said mercaptide rich aqueous alkaline solution to an
oxidation zone and therein treating said mercaptide rich aqueous
alkaline solution with an oxidizing agent in the presence of a
metal phthalocyanine oxidation catalyst at oxidation conditions to
oxidize the mercaptides to liquid disulfides;
(d) separating a major portion of said liquid disulfides from said
aqueous alkaline solution which contains residual disulfides in a
separation zone;
(e) passing said residual disulfide containing aqueous alkaline
solution to a reduction zone and reducing said residual disulfides
to mercaptans at reduction conditions; and,
(f) recycling said mercaptan containing aqueous alkaline solution
to said extraction zone.
In a specific embodiment, the invention provides a process for
treating a sour hydrocarbon stream containing mercaptans which
comprises:
(a) contacting said hydrocarbon stream with an aqueous sodium
hydroxide solution in an extraction zone at a temperature of about
10.degree. to about 100.degree. C. and a pressure from ambient to
300 psig to form a purified hydrocarbon stream and a mercaptide
rich aqueous sodium hydroxide solution.
(b) separating and recovering said purified hydrocarbon stream from
said mercaptide rich aqueous sodium hydroxide solution;
(c) passing said mercaptide rich aqueous sodium hydroxide solution
to an oxidation zone and therein oxidizing said mercaptide to
disulfides with an excess amount of air in the presence of a cobalt
phthalocyanine catalyst which is contained in said mercaptide rich
sodium hydroxide solution at a temperature of 30.degree. to
70.degree. C., and a pressure of 30 to 100 psig.
(d) separating a major portion of said liquid disulfides from said
aqueous sodium hydroxide solution which contains residual
disulfides and the cobalt phthalocyanine catalyst in a separation
zone;
(e) passing said residual disulfide containing aqueous sodium
hydroxide solution to a reduction zone and reducing said residual
disulfides to mercaptans by contacting said disulfides with
hydrogen over a palladium on carbon hydrogenation catalyst;
and,
(f) recycling said mercaptan containing aqueous sodium hydroxide
solution to said extraction zone.
Other objects and embodiments of the present invention encompass
details about particular input hydrocarbon streams, catalysts for
use in the oxidation and reduction steps thereof, mechanics
associated with each of the essential steps thereof, and preferred
operating conditions for each of the essential steps thereof.
DETAILED DESCRIPTION OF THE INVENTION
As heretofore stated, this invention relates to a process for
treating a sour hydrocarbon stream. The sour hydrocarbon stream
which is treated by the process is exemplified by one of the
following: light petroleum gas (LPG), light naphtha, straight run
naphthas, methane, ethane, ethylene, propane, propylene, butene-1,
butene-2, isobutylene, butane, pentanes, etc.
The alkaline solution utilized in the present invention may
comprise any alkaline reagent known to have the capability to
extract mercaptans from relatively low boiling hydrocarbon streams.
A preferred alkaline solution generally comprises an aqueous
solution of an alkali metal hydroxide, such as sodium hydroxide,
potassium hydroxide, lithium hydroxide, etc. Similarly, aqueous
solutions of alkaline earth hydroxides such as calcium hydroxide,
barium hydroxide, magnesium hydroxide, etc. may be utilized if
desired. A particularly preferred alkaline solution for use in the
present invention is an aqueous solution of about 1 to about 50% by
weight of sodium hydroxide with particularly good results obtained
with aqueous solutions having about 4 to about 25 wt. percent
sodium hydroxide.
The catalyst which is used in the oxidation step is a metal
phthalocyanine catalyst. Particularly preferred metal
phthalocyanines comprise cobalt phthalocyanine and iron
phthalocyanine. Other metal phthalocyanines include vanadium
phthalocyanine, copper phthalocyanine, nickel phthalocyanine,
molybednum phthalocyanine, chromium phthalocyanine, tungsten
phthalocyanine, magnesium phthalocyanine, platinum phthalocyanine,
hafnium phthalocyanine, palladium phthalocyanine, etc. The metal
phthalocyanine in general is not highly polar and, therefore, for
improved operation is preferably utilized as a polar derivative
thereof. Particularly preferred polar derivatives are the
sulfonated derivatives such as the monosul derivative, the disulfo
derivative, the tri-sulfo derivative, and the tetra-sulfo
derivative.
These derivatives may be obtained from any suitable source or may
be prepared by one of two general methods (as described in U.S.
Pat. Nos. 3,408,287 or 3,252,890). First, the metal phthalocyanine
compound can be reacted with fuming sulfuric acid; or second, the
phthalocyanine compound can be synthesized from a sulfo-substituted
phthalic anhydride or equivalent thereof. While the sulfuric acid
derivatives are preferred, it is understood that other suitable
derivatives may be employed. Particularly, other derivatives
include a carboxylated derivative which may be prepared, for
example, by the action of trichloroacetic acid on the metal
phthalocyanine or by the action of phosgene and aluminum chloride.
In the latter reaction the acid chloride is formed and may be
converted to the desired carboxylated derivative by conventional
hydrolysis. Specific examples of these derivatives include: cobalt
phthalocyanine monosulfonate, cobalt phthalocyanine disulfonate,
cobalt phthalocyanine trisulfonate, cobalt phthalocyanine
tetrasulfonate, vanadium phthalocyanine monosulfonate, iron
phthalocyanine disulfonate, palladium phthalocyanine trisulfonate,
platinum phthalocyanine tetrasulfonate, nickel phthalocyanine
carboxylate, cobalt phthalocyanine carboxylate or iron
phthalocyanine carboxylate.
The preferred phthalocyanine catalyst can be used in the present
invention in one of two modes. First, it can be utilized in a water
soluble form or a form which is capable of forming a stable
emulsion in water as disclosued in U.S. Pat. No. 2,853,432. Second,
the phthalocyanine catalyst can be utilized as a combination of a
phthalocyanine compound with a suitable carrier material as
disclosed in U.S. Pat. No. 2,988,500. In the first mode, the
catalyst is present as a dissolved or suspended solid in the
alkaline stream which is charged to the regeneration step. In this
mode, the preferred catalyst is cobalt or vanadium phthalocyanine
disulfonate which is typically utilized in an amount of about 5 to
about 1,000 wt. ppm of the alkaline stream. In the second mode of
operation, the catalyst is preferably utilized as a fixed bed of
particles of a composite of the phthalocyanine compound with a
suitable carrier material. The carrier material should be insoluble
or substantially unaffected by the alkaline stream or hydrocarbon
stream under the conditions prevailing in the various steps of the
process. Activated charcoals are particularly preferred because of
their high adsorptivity under these conditions. The amount of the
phthalocyanine compound combined with the carrier material is
preferably about 0.1 to about 2.0 wt. percent of the final
composite. Additional details as to alternative carrier materials,
methods of preparation, and the preferred amount of catalytic
components for the preferred phthalocyanine catalyst for use in
this second mode are given in the teachings of U.S. Pat. No.
3,108,081.
The disulfide reduction step can be accomplished either by
hydrogenation using a hydrogenation catalyst and hydrogen or by
electrochemically reducing the disulfide. Hydrogenation of the
disulfide occurs via the following equation:
In the preferred embodiment of the process the catalyst for the
hydrogenation reaction consists of a metal on a solid support. The
support can be chosen from the group comprising carbon, alumina,
silica, aluminosilicates, zeolites, clays, etc. while the metal is
preferably chosen from the metals of Group VIII of the Periodic
Table and more preferably from the group comprising nickel,
platinum, palladium, etc. The preferred supports are carbon based
due to their stability in strong caustic and include activated
carbons, synthetic carbons, and natural carbons as examples.
Particularly preferred catalysts are: palladium on a carbon support
and platinum on a carbon support.
In general, the palladium or platinum catalysts may be prepared by
methods known in the art. For example, a soluble palladium salt can
be contacted with a carbon support in order to deposite the desired
amount of the palladium salts. Examples of soluble palladium salts
which may be used are palladium chloride, palladium nitrate,
palladium carboxylates, palladium sulfate and amine complexes of
palladium chloride. This catalytic composite can then be dried and
calcined. Finally, the finished palladium catalyst may be activated
by reduction, if desired, by treatment with a reducing agent.
Examples of reducing agents are gaseous hydrogen, hydrazine or
formaldehyde.
The preferred catalyst is used under the following hydrogenation
conditions: a hydrogen concentration of 10 to 100 times the
stoichiometric amount required for the reaction, an LHSV from about
3 to about 18, and a temperature from about 30.degree. C. to about
150.degree. C. Preferred reaction conditions are a hydrogen
concentration of 50-100 times the stoichiometric amount, a LHSV
from about 6 to 12 and a temperature from about 50.degree. C. to
about 100.degree. C.
Alternatively the disulfide can be reduced by electrochemical
means. The electrochemical cell which may be employed to effect the
reduction step in the present process consists of a cathode and an
anode electrode, and an electrolytic solution. The cathode
electrode may be chosen from the group of metals comprising zinc,
lead, platinum, graphite, glossy carbon, synthetic carbons,
cadmium, palladium, iron, nickel, copper, etc. while the anode
electrode may be chosen from the group comprising platinun,
graphite, iron, zinc, and brass electrode. The electrodes may also
consist of a combination of the above metal systems, for example
zinc coated graphite, or platinum coated graphite. The electrolytic
solution is the disulfide containing alkaline solution itself. When
a voltage is applied to the two terminals, the following reactions
occur at the electrodes: ##EQU1## The anode reaction is not limited
to the oxidation of water and, in principle, may be any suitable
oxidation which can be coupled with the disulfide reduction
reaction of complete the electrochemical reaction. This
electrochemical process can be done either as a batch process or as
a continuous process, with the continuous process being preferred.
A voltage from about 1.3 v to about 3.0 v is applied with the
preferred voltage being from about 1.5 v to about 2.5 v.
BRIEF DESCRIPTION OF THE DRAWING
This invention will be further described with reference to the
attached drawing which is schematic outline of the process under
discussion. The attached drawing is merely intended as a general
representation of a preferred flow scheme with no intent to give
details about vessels, heaters, condensers, pumps, compressors,
valves, process control equipment, etc. except where a knowledge of
these devices is essential to the understanding of this invention
or would not be self-evident to one skilled in the art.
Referring now to the attached drawing, a hydrocarbon stream enters
the process via line 1 into extraction zone 3. The aqueous alkaline
solution containing the phthalocyanine catalyst enters the process
via line 2 into extraction zone 3. Extraction zone 3 is typically a
vertically positioned tower containing suitable contacting means
such as baffle pans, trays, and the like designed to effect
intimate contact between the two liquid streams charged thereto. In
extraction zone 3 the sour hydrocarbon stream is counter-currently
contacted with an alkaline solution containg a phthalocyanine
catalyst which enters the extraction zone via line 2. When desired,
fresh alkaline solution may be introduced into the system by an
extension of line 2.
The function of extraction zone 3 is to bring about intimate
contact between the sour hydrocarbon stream and the alkaline stream
such that the mercaptans contained in the hydrocarbon stream are
preferentially dissolved in the alkaline solution. The rate of flow
of the sour hydrocarbon stream and the alkaline solution are
adjusted so that the treated hydrocarbon stream leaving the
extraction zone 3 via line 5 contains substantially less mercaptans
than the sour hydrocarbon stream introduced via line 1. In this
manner zone 3 acts to both extract the mercaptans from the sour
hydrocarbon stream into the alkaline solution and to separat the
treated hydrocarbon stream from the alkaline solution.
Extraction zone 3 is preferably operated at a temperature of about
25.degree. to about 100.degree. C. and more preferably at a
temperature of about 30.degree. to about 75.degree. C. Likewise,
the pressure utilized within zone 3 is generally selected to
maintain the hydrocarbon stream in liquid phase, and may range from
ambient up to about 300 psig. For an LPG stream the pressure is
preferably about 140 to about 175 psig. The volume loading of the
alkaline stream relative to the hydrocarbon stream is preferably
about 1 to about 30 vol. percent of the hydrocarbon stream with
excellent results obtained for an LPG type stream when the alkaline
stream is introduced into zone 3 in an amount of about 5% of the
hydrocarbon stream.
The mercaptide rich alkaline stream is passed via line 4 to
oxidation zone 6 where it is commingled with the oxidant which
enters the oxidation zone 6 via line 7. The amount of oxidant such
as oxygen or air commingled with the alkaline stream is ordinarily
at least the stoichiometric amount necessary to oxidize mercaptides
contained in the alkaline stream to disulfides. In general, it is a
good practice to operate with sufficient oxidant to ensure that the
reaction goes essentially to completion. The oxidant used for this
step comprises an oxygen-containing gas such as oxygen or air with
air usually being the oxidant of choice for economic and
availability reasons. The function of zone 6 is to regenerate the
alkaline solution by oxidizing the mercaptide compounds to
disulfides; as pointed out hereinbefore, this regeneration step is
preferably performed in the presence of a phthalocyanine catalyst
which is present as a solution in the alkaline stream. In the
preferred embodiment of the apparatus, a suitable packing material
is utilized in order to effect intimate contact between the
catalyst, the mercaptides and oxygen.
Zone 6 is preferably operated at a temperature corresponding to the
temperature of the entering mercaptide rich alkaline solution which
is typically in the range of about 35.degree. to about 70.degree.
C. The pressure used in zone 6 is generally substantially less than
that utilized in the extraction zone. For instance, in a typical
embodiment wherein extraction zone 3 is run at a pressure from
about 140 to about 175 psig, zone 6 is preferably operated at about
30 to about 70 psig.
An effluent stream containing nitrogen, disulfide compounds,
alkaline solution and optionally phthalocyanine catalyst is
withdrawn therefrom via line 8 and passed to a separating zone 9
which is preferably operated at the conditions used in zone 6. In
zone 9 the effluent stream is allowed to separate into (a) a gas
phase which is withdrawn via line 10 and discharged from the
process, (b) a disulfide phase which is substantially immiscible
with the alkaline phase and is withdrawn from the process via line
11 and (c) an alkaline phase which is withdrawn via line 12. In
general, the complete coalescence of the disulfide compound into a
separate phase is extremely difficult to achieve without the aid of
suitable coalescing agents such as a bed of steel wool, sand,
glass, etc. In addition, a relatively high residence time of about
0.5 to 2 hours is typically used within zone 9 in order to further
facilitate this phase separation. Despite these precautions, the
regenerated alkaline stream which is withdrawn via line 12
inevitably contains minor amounts of disulfide compounds and
mercaptide compounds. In fact, the amount of sulfur present in this
regenerated alkaline stream can build up during the course of a
prolonged recycle operation such that complete treatment of the
sour hydrocarbon stream in extraction zone 3 is not possible.
In accordance with the present invention, the regenerated alkaline
solution is passed to zone 13 via line 12. The function of zone 13
is to reduce the disulfides entrapped in the alkaline solution.
Zone 13 can be configured in one of two configurations: a catalytic
hydrogenation or an electrochemical reduction configuration.
In the catalytic hydrogenation configuration, zone 13 preferably
contains a fixed bed catalyst of 10-30 mesh particles comprising
palladium on carbon. Hydrogen is charged to zone 13 via line 15 and
intermingled with the alkaline solution in contact with the
hydrogenation catalyst thereby reducing the disulfides to
mercaptides. This zone is preferably operated at a temperature of
about 30.degree. C. to about 150.degree. C., a pressure of about 30
psig to about 150 psig, an LHSV of about 1 to about 20 and a
hydrogen concentration of about 10 to about 100 times the
stoichiometric amount. In the preferred embodiment of the invention
the reduction conditions will include a temperature of about
40.degree. C. to about 100.degree. C., an LHSV of about 3 to about
15, a pressure of about 50 psig to about 125 psig and a hydrogen
concentration of about 15 to about 30 times the stoichiometric
amount. The effluent stream is separated into an unreacted hydrogen
gas phase which is withdrawn via line 14 and discharged from the
process and an alkaline aqueous phase which is withdrawn via line
16, joined to line 2 and cycled to extraction zone 3.
Alternatively the hydrogenation catalyst can comprise a soluble
hydrogenation catalyst, such as a Group VIII carboxylate, and be
present in the alkaline solution throughout the entire process. In
this case, zone 13 is preferably operated at a temperature of about
30.degree. C. to about 125.degree. C., a pressure of about 30 psig
to about 150 psig, a residence time of about 3 mw to about 30 mw
and a hydrogen concentration of about 10 to about 100 times the
stoichiometric amount. In the preferred embodiment of the invention
the reduction conditions will include a temperature of about
40.degree. C. to about 100.degree. C., an LHSV of about 3 to about
15, a pressure of about 50 psig to about 125 psig and a hydrogen
concentration of about 15 to about 30 times the stoichiometric
amount.
In the electrochemical configuration, zone 16 comprises an
electrochemical cell consisting of a cathode, an anode and an
electrolytic solution. The electrolytic solution is the
to-be-treated alkaline solution which is introduced into zone 13
via line 12. The cathode electrode of the cell is preferably
graphite. The anode electrode is preferably platinum or graphite.
This electrochemical reduction can be carried out either as a batch
process or a continuous process. A voltage from about 1.3 v to
about 3.0 v is applied with the preferred voltage being from about
1.5 v to about 2.5 v. When operated as a batch process, the
residence time is preferably about 30 min to about 240 min, while
when operated as a continuous process a residence time of about 3
min to about 30 min is preferred. As in the catalytic hydrogenation
reduction, the effluent stream separates into a gas phase,
primarily comprising oxygen which is withdrawn via line 14 and an
alkaline aqueous phase which is withdrawn via line 16, joined to
line 2 and cycled to extraction zone 3.
The following examples are given to illustrate further the process
of the present invention, and indicate the benefits to be afforded
by the utilization thereof. In particular the examples describe
only the reduction part of the invention. It is understood that the
examples are given for the sole purpose of illustration and are not
considered to limit the generally broad scope and spirit of the
appended claims.
EXAMPLE 1
A palladium on carbon hydrogenation catalyst was prepared in the
following manner. To a beaker containing 500 mL of deionized water
was added 7.5 grams of palladium nitrate, Pd (NO.sub.3).sub.2
.times.H.sub.2 O. In a separate beaker 200 grams (450 mL) of 10-30
mesh carbon was wetted with 450 mL of deionized water. The
palladium nitrate solution and the wetted carbon were mixed in a
rotary evaporator and rolled for about 15 minutes. After this
period of time, the evaporator was heated by introducing steam into
the evaporator so that the aqueous phase was evaporated. The
complete evaporation of the aqueous phase took about 3 hours. Next
the impregnated catalyst was dried in a forced air oven for 3 hours
at 80.degree. C. Finally the dried catalyst was then calcined under
nitrogen at 400.degree. C. for 2 hours. The final catalyst
composite contained 1.13% Pd by weight.
A commercial alkaline solution having a disulfide content of 298
wt. ppm was contacted with the 10-30 mesh fixed bed palladium on
carbon catalyst described above at an LHSV of 10, a temperature of
75.degree. C., a pressure of 100 psig and a hydrogen concentration
of 80 times the stoichiometric amount. After three hours, the
effluent was analyzed for disulfides and it was determined that 74%
of the disulfides were being converted to mercaptans. The feed
stream was continuously fed through the reaction vessel containing
the catalyst at the conditions stated herein for 110 hours at which
point the conversion of disulfide to mercaptan was found to be
90%.
Clearly this process is effective in reducing the disulfides to
mercaptans at a high yield. Therefore, the instant invention
significantly reduces the disulfide content of the alkaline stream
recycled to the extraction zone described hereinbefore.
EXAMPLE II
A zinc cathode electrode and a platinum anode electrode were placed
in a 500 ml beaker. 300 ml of a 6.0% sodium hydroxide solution
containing 300 wt. ppm disulfide were added to the beaker and a
voltage of -1.8 V was applied across the two electrodes. After 4
hours the solution was analyzed for disulfides and it was
determined that 53% of the disulfides were converted to
mercaptans.
It is observed that the electrochemical reduction of disulfides to
mercaptans using a zinc cathode electrode is an effective way to
minimize the entry of disulfides into the extraction zone.
EXAMPLE III
A lead cathode electrode and a platinum anode electrode were placed
in a 500 ml beaker. 300 ml of a 6.0% sodium hydroxide solution
containing 300 wt. ppm disulfide were added to the beaker and a
voltage of -1.8 V was applied across the two electrodes. After 4
hours the solution was analyzed for disulfides and it was
determined that 39% of the disulfides were converted to
mercaptans.
It is observed, therefore, that the electrochemical reduction of
disulfides to mercaptans using a lead cathode electrode is an
effective way to minimize the entry of disulfides into the
extraction zone which would increase the total sulfur content of
the treated hydrocarbon stream.
EXAMPLE IV
A graphite rod cathode electrode and a platinum anode electrode
were placed in a 500 mL beaker. To this beaker there was added 300
mL of a 6.0% sodium hydroxide solution containing 300 wt. ppm of
disulfide and a voltage of -1.8 v was applied across the two
electrodes. After a 6 hour period 25% of the disulfides were
converted to mercaptans.
It is observed, therefore, that the electrochemical reduction of
disulfides to mercaptans using a graphite electrode is an effective
way to minimize the entry of disulfides into the extraction
zone.
In addition, carbon based electrodes such as graphite show very
high stability to strongly alkaline solutions, making carbon based
electrodes the preferred material for the cathode electrode.
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